A defect usually. Disabilities disable someone from doing everything a person without disabilities can do. They range from Paralyzation, Down Syndrome, Autism, ADD, and many, many more. Not all disabilities are birth defects, one can be injured in an event and be disabled from then on or until cured.

Ted: Man, this damn truck hit me and now I can't walk for a while.
Bob: Well I can't ever walk, so fuck you.